Magnetorheological fluids that comprise suspensions of magnetic particles such as iron or iron alloys in a fluid medium are well known. The flow characteristics of these fluids can change by several orders of magnitude within milliseconds when subjected to a suitable magnetic field due to suspension of the particles. The ferromagnetic particles remain suspended under the influence of magnetic fields and applied forces. Such magnetorheological fluids have been found to have desirable electromagnetorheological interactive properties for advantageous use in a variety of controllable coupling and damping devices, such as brakes, clutches, and dampers.
In particular, linear acting MR dampers are proposed for suspension systems, such as a vehicle suspension system and vehicle engine mounts. PCT patent application 10840, published Jan. 8, 1998 (the '840 application), discloses a conventional linear acting controllable vibration damper apparatus that includes a piston positioned in a magnetorheological fluid-filled chamber to form upper and lower chambers. The piston includes a coil assembly, a core, i.e. pole pieces, and an annular ring element positioned around the pole pieces to form an annular flow passage for permitting flow of the magnetorheological fluid between the chambers. When the piston is displaced, magnetorheological fluid is forced through the annular flow passage. When the coil is energized, a magnetic field permeates the channel and excites a transformation of the magnetorheological fluid to a state that exhibits increased damping forces.
The damping performance of a suspension damper is largely dependent on the force-velocity characteristics of the damper. In standard suspension dampers of the prior art that do not use MR fluid, the force-velocity curve typically has a steeper slope at low velocities and desirably passes through the zero point of damping force at zero velocity, thus producing a smooth transition between damper movements in compression and extension directions. Without special design considerations, however, a suspension damper using MR fluid tends to have a force-velocity curve that intersects the force axis at a value above zero from the positive velocity side, as seen in curve 50 of FIG. 4, and a value below zero from the negative velocity side, thus producing a jump in force between finite positive and negative values with each change in the direction of damper movement. These jumps in force tend to provide a harshness to the vehicle ride which may be felt by the vehicle occupants. Conventional MR dampers attempt to solve the zero intersect problem by including one or more fluid bypass passages through the piston or on the outer surface thereof, in an area of weak or no magnetic flux and not open to the main, magnetic flux controlled fluid path through the piston: for example in the outer surface of the flux ring. The relatively unimpeded flow of MR fluid through the outer bypass passages permits the damping curves to intersect zero. However, this design also results in an undesirable steep rise in the damping curve from the zero point followed by a sharp transition into higher velocities. In addition, the steep rise may often result in the damper overshooting the desired force at the transition. The steep slope and overshooting, as seen in curve 52 of FIG. 4, results in discontinuities that are generally undesirable in vehicle suspensions. Specifically, the use of a totally separate bypass passage impairs the ability to achieve noise control and smooth load transfer. Also, the MR fluid flowing through the outer bypass passages is not within the magnetic flux path, is not exposed to magnetic flux and therefore, does not experience an MR effect. As a result, the outer passages represent a pure loss in pressure in the system that disadvantageously reduces the maximum force achievable.
Therefore, there is a need for an MR damper capable of effectively providing a smooth and controllable transition, without a sharp break in the damper force/velocity curve, between very low damping forces near zero damper piston velocity to higher damping forces at higher damper piston velocities while maintaining desirable maximum force levels.